Add support for paragonie/constant_time_encoding v3

[wtsergo]

Hello,

Can you add support for paragonie/constant_time_encoding v3 ?
I think change in composer.json from
"paragonie/constant_time_encoding": "^2.2"
to
"paragonie/constant_time_encoding": "^2 || ^3"
will be enough because signature for "ParagonIE\ConstantTime\Base64UrlSafe::encode" is not changed.

Thanks,
Serhii

[kelunik]

Do you want to provide a PR?

[wtsergo]

#18